Technical Field
The present disclosure relates to an improved vacuum integrated electronic device and the manufacturing process thereof.
Description of the Related Art
As is known, the idea of miniaturized vacuum integrated electronic devices dates back to 1961. However, the increasing demand for high-speed high-power telecommunication systems has recently given a new impulse to the research in the field of vacuum micro- and nanoelectronics because of their good characteristics in handling high voltages and high powers.
Therefore, the availability of a device merging the above advantages with those of the solid-state technology would open new potential scenarios of future markets and products for long-range telecommunications, aerospace and medical systems.
Unfortunately, manufacturing such devices has proven to be difficult, in particular as regards the shape and surface composition of the cathode. Specifically, techniques to concentrate the electric field with sufficient intensity to produce emission with practical turn-on voltages have been studied and materials with low-work function, good chemical, thermal, mechanical and electrical properties have been investigated. These materials must also be suitable for the implementation of tips with small radius and high aspect ratio (the ratio between the base diameter and the tip height).
Generally, these structures have conical or pyramidal metal micro-tip cathodes.
For example, FIG. 1 shows the structure of a triode having a conical tip. The triode of FIG. 1 comprises first, second and third metal layers 2, 3, 4, separated by dielectric layers 5, 6, deposited on a glass substrate 1. The first metal layer 2 extends on the glass substrate 1 and forms a cathode; the second metal layer 3 extends between the first and the third metal layers 2, 4 and forms a gate and the third metal layer 4 forms an anode. A cavity 8 is formed in the dielectric layers 5, 6 and in the second metal layer 3 and houses a tip 9, extending from the first metal layer 2 toward the second metal layer 3.
Here, emission is caused by the gate-cathode voltage and emitted electrons are collected by the anode 4.
This solution is quite complex to be manufactured.
Another known solution is a lateral structure, which can be fabricated in a planar way, as shown in FIG. 2. Here, an anode region 10, a cathode region 11 and two gate regions 12 are formed in a single metal layer and are shaped to obtain a controlled emission of electrons. The lateral structure offers a simpler fabrication process and easy shaping of electrodes through lithography, but at the expense of large area occupation and reduced current density.
In another possible structure, shown in FIG. 3, electron emission does not originate from a small tip region but, rather, from a peripheral edge 14 of a thin metal cathode region 20, that is holed. An anode region 21 and a gate region 22 are similar to those in FIG. 1. The main drawback of this solution is the high area occupation.
In an alternative structure, disclosed in U.S. Pat. No. 5,463,269, a vacuum integrated microelectronic device is manufactured by conformal deposition of an insulating material in a cavity, thus forming a symmetrical cusp that can be used as a mold to form a micro-tip cathode. Two electrodes form a simple diode, while three, four or five electrodes can form, respectively, a triode, a tetrode and a pentode. Since the cusp is self-aligned to the center of the cavity, it is also aligned to the center of the electrodes. However, the manufacture of the above vacuum integrated microelectronic device has high manufacturing costs and its operating characteristics can be altered by, for instance, ionizing radiations and noise at power output.
MI2013A000897 (US 2014/0353576) describes an electron emitting device wherein the cathode is formed by depositing a metal layer on a dielectric layer having a cavity. During deposition, the metal material forms horizontal portions that protrude over the cavity and joins to form a tip. The width of the cavity is such that the metal layer does not fall into the cavity, which is thus sealed by the metal layer.
Although this solution has proved satisfactory in many situations, it cannot be used in all applications and devices. In fact, the voltage causing turning on of the electron emission is quite high, e.g., up to 20V, which is too high for some electronic application. In addition, this voltage is far from common threshold voltage of components integrated in VLSI/ULSI technology. Thus, a better compatibility with voltages used in common semiconductor voltages is desired.
Thus, an aim of the disclosure is to provide an improved vacuum integrated electronic device.